The present invention relates to refrigerated display cases employing ambient air defrost and a method of operating such cases. Both within the specification and the claims of the present application, all references to refrigeration apparatus or refrigeration operations are intended to include both cooling at a temperature below 32.degree. F., such as associated with frozen food display cases, and in excess of 32.degree. F., such as typically associated with diary food and fresh meat display cases.
In the operation of all types of refrigerated display cases, it is desirable to include a system capable of automatically defrosting the display case. The defrost cycle can be actuated either at set periodic times or when the frost buildup within the system has reached a certain predetermined level. Such systems are typically thermostatically controlled so as to switch from a refrigeration cycle to a defrost cycle of operation. By this manner of operation, it is possible to avoid any significant frost buildup within the display case.
Typically within the prior art, there have been three different approaches employed for defrosting refrigerated display cases. The first approach involves the use of electric resistance heaters that are arranged adjacent to the refrigeration coils of the refrigeration mechanism. During a defrost cycle, these heaters supply heat in an effort to eliminate the frost buildup on the coils; however, the heaters also add warmer air to the air conduit for circulation within the case. During such a defrost cycle, the fans for circulating air through the primary air conduit, i.e. the conduit in which the coils are located, can be turned off as disclosed in U.S. Pat. No. 3,756,038 to MacMaster et al. The particular technique is relatively simple both in its construction and operation. However, the electrical heaters are high wattage heaters that utilize significant electricity during operation. Furthermore, the warm air circulated in the case can raise the temperature of the case too high. Thus, attempts have been made to find alternatives to such a system.
A second type of system circulates compressed gaseous refrigerant through the refrigeration coils during the defrost cycle. During the defrost cycle, a valve control mechanism shuts off the supply of refrigerant to the refrigeration coils and alternatively feeds compressed gaseous refrigerant through the coils. This gas serves to reduce any frost buildup that has occurred on the refrigeration coils but simultaneously provides heat within the air conduit which can be circulated through the display case, which again is disadvantageous. Due to the requirement that the system be able to selectively switch between the supply of the gas for defrosting and refrigerant to the refrigeration coils, a valving structure must be provided. Such a mechanism increases the cost of construction of the system. In addition, the provision of such a system increases the number of parts capable of breaking down thereby necessitating costly repairs.
The third type of system employed for defrosting display cases relies upon ambient air. It is this general category with which the invention of the present application is concerned. One type of system that employs ambient air during the defrost cycle is exemplified by those embodiments illustrated in U.S. Pat. Nos. 3,403,525, 3,850,003 and 3,937,033, all to Beckwith et al. These systems use fans separate and distinct from the main air circulating fans. These extra fans are only turned on during the defrost cycle for pulling ambient air from outside of the display case directly into the air conduits. A second type of system is illustrated in U.S. Pat. No. 3,082,612 to Beckwith, which system draws ambient air into the main circulation path through ports located in the lower front panel of the refrigerated display case. Such ports are normally closed during the refrigeration cycle and are opened during the defrosting cycle. The Beckwith et al. U.S. Pat. No. 3,850,003 indicates that the concepts described in U.S. Pat. Nos. 3,082,612 and 3,403,525 did not prove to be practical and hence were not commercially feasible.
Finally, a third type of ambient air defrosting system is shown in U.S. Pat. No. 4,144,720 to Subera et al., which is assigned to the same assignee as the present application. In the foregoing patent application, an open front refrigerated display case having primary and secondary air conduits is disclosed. In this system, the direction of airflow within one of the conduits is reversed, for example by the use of reversible fans, thereby drawing in air from outside of the display case. Two other patents disclosing the use of reversible fans for ambient air defrost are U.S. Pat. No. 4,026,121 to Aokage and U.S. Pat. No. 4,120,174 to Johnston.